Method for inducing hearing

ABSTRACT

There is disclosed a method for inducing the sensation of intelligible hearing by direct electrical excitation of the auditory nerve endings distributed along the basilar membrane within the cochlea. An electrode is positioned within the lower scala of the cochlea by insertion through the round window. The electrode consists of a resilient base member shaped to conform to the inner surface of the lower scala, such base member extending along the basilar membrane. The base member retains a pair of conductors which extend parallel to the length of the basilar membrane. An electrical excitation signal corresponding to an externally generated audio signal is conducted to the conductors of the electrode thereby generating a uniform, alternating electrical field along the basilar membrane which replaces the naturally generated auditory electrical field.

United States atent 1 1 Michelson 51 Aug.'7,1973

1 1 METHOD FOR INDUClNG HEARHNG Robin P. Michelson, Redwood City, Calif.

22 Filed: Feb.4, 1972 21 Appl. No.: 223,416

Related U.S. Application Data {63] Continuation-in-part of Ser. No.75,076, Sept. 24

1970, abandoned.

175] inventor:

OTHER PU BLlCATlONS The Crossed Cochlea Effect by Michelson, X-actionsof American Largngological, Rhinological & Othological Society, lnc.,Presented 1/3/68, Publication Received Library of Medicine 1/8/69.

Primary Examinerl(athleen H. Claffy Assistant ExaminerThomas D'AmicoA!!0rneyR. J. Steinmeyer and James M. Thomas [57] ABSTRACT There isdisclosed a method for inducing the sensation of intelligible hearing bydirect electrical excitation of the auditory nerve endings distributedalong the basilar membrane within the cochlea. An electrode ispositioned within the lower scala of the cochlea by insertion throughthe round window. The electrode consists of a resilient base membershaped to conform to the inner surface of the lower scala, such basemember extending along the basilar membrane. The base member retains apair of conductors which extend parallel to the length of the basilarmembrane. An electrical excitation signal corresponding to an externallygenerated audio signal is conducted to the conductors of the electrodethereby generating a uniform, alternating electrical field along thebasilar membrane which replaces the naturally generated auditoryelectrical field.

9 Claims, 9 Drawing Figures p f A M %,,gg; 440004470? D 76 75dim/0004,4701? Rizal/5e 056/1- 4 4704/ TEA/V5M/7'7'E/Q 1 //V7'/4 C061624148 5.: cree s METHOD FOR INDUCING HEARING CROSS REFERENCE TORELATED APPLICATION This application is a continuation-in-part of mycopending application Ser. No. 75,076, filed Sept. 24, 1970 nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a method for inducing hearing and, moreparticularly, to a method for inducing the sensation of intelligiblehearing, by direct electrical excitation of the auditory nerve endingsdistributed along the basilar membrane within the cochlea, in peoplesuffering from sensory deafness. This condition is untreatable byacoustic amplification or bone conduction.

2. Description of the Prior Art The fundamentals of the hearing process,whereby the vibrations of the surrounding air called sound are sensed bythe auditory system and transmitted to the brain, are well defined. Forpresent purposes, such hearing process may be briefly described asfollows: The auditory system may be divided into its three componentparts, namely the external, the middle and the internal ear. Theexternal ear is outermost and includes the auricle attached to the sideof the head and the external auditory meatus. Sound vibrations in theair are focused by the auricle and conveyed to the opening of theexternal ear canal which transmits such vibrations to the tympanicmembrane which seals the inner end of the auditory meatus and forms thedividing line between the external and middle ears.

The middle ear is positioned within a space in the temporal bone of theskull and serves to transmit the vibratory movements of the tympanicmembrane to the internal ear. The middle ear includes a series of bonescalled the auditory ossicles" which include the mallens, or hammer; theincus, or anvil; and the stapes or stirrup. The hammer is directlyattached to the tympanic membrane whereas the stirrup is attached to amembrane positioned in a minute opening, called the oval window in thebony area containing the internal ear. The auditory ossicles areinterconnected so that the vibratory movements of the tympanic membraneare transmitted to the oval window, and sound is thus transmitted fromthe external to the inner ear.

The portion of the inner ear specifically concerned with hearingconsists of the cochlea, a long, narrow duct within the temporal bone,which is wound spirally around its axis for approximately two andone-half turns. The cochlea is divided by a pair of membranes extendinglongitudinally therethrough into an upper, a middle and a lower scala.The oval window presents an opening into the upper scala, an additionalminute opening, called the round window," providing an opening into thelower scala, the round window being closed by a membrane. The cochlea isfilled with a fluid, the perilymph, which is free to circulate throughthe upper and lower scalas which are interconnected at the apex of thecochlea.

The membrane between the middle scala and the lower scala, called thebasilar membrane, extends the entire length of the cochlea duct. Theauditory pathways from the cochlea terminate in the cerebral cortex ofthe brain. The auditory nerve endings, distributed along the basilarmembrane, are in direct functional connection with the hair cellscontained in the middle scala.

The vibratory movements of the tympanic membrane which are transmittedby the auditory ossicles to the oval window are distributed through thecochlea fluid throughout the cochlea. This vibratory input manifestsitself in an alternating electrical field within the structure of thecochlea (which electrical field appears and has been detected at theround window). This electrical field (generated by the hair cells) issensed by the nerve endings in the basilar membrane and transmitted viathe auditory nerve to the cerebral cortex of the brain which interpretssuch electrical signals as sound.

The loss of hearing, or a decrease in hearing sensitivity, may resultfrom damage or abnormalities in the ex ternal, the middle or theinternal ear. Where the hearing problem is a loss of sensitivity, theproblem is usually solved by the use of a conventional hearing aid whichsimply amplifies the sound before transmission to the tympanic membrane.On the other hand, where hearing sensitivity is reduced to a point whereadditional amplification or bone conduction is useless, suchconventional hearing aids are incapable of generating the sensation ofhearing.

Where total loss of hearing is due to malfunctions in the external ormiddle ear, such as a stiffening of the tympanic membrane or an improperfunctioning of the auditory ossicles, hearing can usually be restoredthrough surgical procedures whereby either the tympanic membrane or oneor more of the auditory ossicles are replaced by man-made or humansubstitutes. However, the total loss of hearing as a result ofdifficulty in the external or middle ear represent a minority of actualcases. The majority of instances of total loss of hearing results fromeither sensory or neural deafness. In the former case, deafness resultsfrom a reduction in the sensitivity of the cochlea in the internal carwhich may be caused, for example, from a loss of hair cells, a chemicalchange in the perilymph, etc. In the latter case, deafness results fromdamage to the auditory nerve itself, either through disease or physicalrupture. In either case, where total deafness results, such that aconventional amplifying hearing aid is useless, no technology presentlyexists for successfully restoring the sensation of hearing.

Regarding the prior art which is considered to be relevant to theinstant invention, reference is made to my article entitled The CrossedCochlea Effect," published in the Transactions of the AmericanLaryngological, Rhinological and Otoligical Society, pp. 626 to 644,1968. In this publication I describe experiments I conducted todetermine the extent of auditory reflexes in cats in order to obtain abetter understanding of certain auditory functions and theirinteractions. In the experiments an audio signal (sound pressure signal)was applied to one ear of each cat and monitored by a cochlearmicrophonic electrode in the same car. The signal was modified by anelectrical stimulation in the contralateral ear. The purpose was todetermine the levels of electrical stimulation in the contralateral earwhich might suppress the cochlear microphonic signal in the acousticallystimulated ear. These levels were found to be approximately 250microvolts to 2 millivolts. The center frequency of the tuning curveindicated a minimum stimulation or threshold level for acoustic reflexin the range of 250 to 500 microvolts. These stimulation levels are ofthe same order as the cochlear microphonic, i.e., the naturallygenerated electrical signal within the cochlea. Through theseexperiments it was demonstrated that there is an acoustic reflexinteraction between the two ears of a cat. The acoustic reflex, however,is not the same as the sensation of hearing. The experiments, therefore,did not demonstrate that the cats were actually hearing an audio signal.In fact, as will appear from the description hereinafter, the electricaland sound pressure stimuli used in these experiments was well below theminimum perception threshold required for the cats to hear.

The present invention involves the use of electrical stimulation of theauditory organ to produce hearing in the deaf. Reference is made to myarticle entitled Electrical Stimulation of the Human Cochlea in SensoryDeafness" published in Archives of Otolaryngology, March 1971, Vol. 93,pp. 317-323, which describes the efforts of other scientists prior tothe present invention to produce hearing by electrical stimulation ofthe auditory organ, as well as results achieved with the use of thepresent invention. This article refers to an implanted electrode systemdeveloped by James H. Doyle, which system is described in detail in U.S.Pat. No. 3,449,768. Doyle utilizes what he calls a neural potentialgenerator which produces 1 KI-Iz clock pulses modulated in amplitude andwidth to create a complex modulation scheme which is intended toduplicate the firing rates and potentials of the neurons along thebasilar membrane. A complex electrode is utilized consisting of amultiplicity of wires driven from a subcutaneous transformer in aunipolar manner from ground plane to the individual electrode wires. Theelectrode is so dimensioned that once it is inserted in the lower scalait is free to move therein. Thus, the pulses produced by the neuralpotential generator are distributed in a random fashion along thebasilar membrane without regard for the place frequency relationship.The place frequency relationship first discovered by Von Bekesy, simplystates that particular portions along the basilar membrane are relatedto specific frequencies. The area of the basilar membrane closest to theround window is associated with the low frequencies. Doyle states thathis patients heard the carrier frequency produced by the neuralpotential generator. The Doyle system has not been successful ininducing the sensation of intelligible hearing.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis disclosed a method for inducing the sensation of intelligible hearingbydirect electrical stimulation of the auditory nerve endings of theauditory nerve. Since the present technique completely by-passes theexternal and middle ears and the hair cells of the inner ear, it ispossible to induce the sensation of intelligible hearing in the absenceof these structures. Thus, the present invention may be effectively usedto induce the sensation of hearing in people suffering from deafnesscaused by abnormalities in any of these areas. However, the primary usewill be in cases of sensory deafness which has, heretofore, beenuntreatable.

Briefly, the sensation of hearing is induced by positioning an electrodewithin the lower scala of the cochlea, such electrode being surgicallyinserted through the round window. The electrode consists of a resilientbase member shaped to conform to the inner surface of the lower scala,such base member extending along the basilar membrane. The base memberretains a pair of conductors which extend parallel to the length of thebasilar membrane. An electrical excitation signal corresponding to anexternally generated audio signal is conducted to the conductors. Theexcitation signal creates a uniform, alternating electrical fieldbetween the conductors. This field is transmitted through the conductivecochlea fluid to the nerve endings in the basilar membrane, thusreplacing the naturally generated auditory electric field.

It is therefore an object of the present invention to provide a methodfor inducing hearing.

It is a further object of the present invention to provide a method forinducing the sensation of hearing in individuals suffering from total ornear total sensory deafness.

It is a still further object of the present invention to provide amethod for inducing the sensation of hearing by electrical stimulationof the nerve endings of the auditory nerve.

It is another object of the present invention to provide a method forinducing the sensation of hearing by positioning an electrode within thelower scala of the cochlea and conducting electrical excitation signalsto such electode so as to directly stimulate the nerve endings of theauditory nerve distributed along the basilar membrane within thecochlea.

Still other objects, features and attendant advantages of the presentinvention will become apparent to those skilled in the art from areading of the following detailed description of the preferredembodiment constructed in accordance therewith, taken in conjunctionwith the accompanying drawings wherein like numerals designate likeparts in the several figures and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showng thefundamental elements of the hearing process;

FIG. 2 is an enlarged, front elevation view, partly in.

section, of the human cochlea;

FIG. 3 is an enlarged, cross-sectional view taken along the line 3-3 inFIG. 2;

FIG. 4 is an enlarged, front elevation view of a preferred embodiment ofintra-cochlear electrode;

FIG. 5 is a cross-sectional view taken along the line 5-5 in FIG. 4;

FIG. 6 is a cross-sectional view of the lower scala of the cochlea,similar to FIG. 3, showing the intracochlear electrode of FIGS. 4 and Sin place;

FIG. 7 is a block diagram of a preferred embodiment of apparatus forexciting an intra-cochlear electrode;

FIG. 8 is a circuit diagram of a preferred embodiment of the receivingelements of the circuitof FIG. 7; and

FIG. 9 is a view showing the physical configuration of a preferredembodiment of electrode and receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand, more particularly, to FIG. 1 thereof, there is shown, in blockdiagram form, the fundamental elements of the hearing process. Soundvibrations caused by an external sound pressure generator 10 set the airin motion producing spherical pressure waves 11. Pressure waves 11 arecaught by the external ear 12 and transmitted to the tympanic membrane13 which is displaced in response to such waves. The vibratory movementsof the tympanic membrane 13 are transmitted via the auditory ossicles 14to the oval window of the cochlea 15. Hair cells within cochlea 15function as a transducer to generate an alternating electrical fieldwithin cochlea 15. This electrical field is sensed by the nerve endingsdistributed through the basilar membrane and transmitted via theauditory nerve 16 to the cerebral cortex 17 of the brain, whichinterprets such electrical signals as sound.

Referring now to FIGS. 2 and 3, the cochlea 15 is a long, narrow ductwithin the temporal bone which is wound spirally arond its axis forapproximately 2% turns. The cochlea is divided by a pair of membranes 21and 22, extending longitudinally therethrough, into an upper scala 23, amiddle scala 24 and a lower scala 25. The oval window 26, which issealed by a membrane in contact with the stirrup of the auditoryossicles, presents an opening into upper scala 23. The round window 27,which is also closed by a membrane, provides an opening into lower scala25. Cochlea 15 is filled with a fluid, the perilymph, which is free tocirculate through upper scala 23 and lower scala 25 which areinterconnected at the apex 28 of cochlea 15.

Membrane 21 between middle scala 24 and lower scala 25, called thebasilar membrane, extends the entire length of cochlea 15. The auditorynerve 16 from the brain terminates in cochlea 15 and the nerve endingsare distributed along basilar membrane 21. Minute hair cells 29 extendfrom basilar membrane 21 into middle scala 24.

The vibratory movements of tympanic membrane 13, which are transmittedby auditory ossicles 14 to oval window 26, are distributed through thecochlea fluid throughout cochlea 15. This vibratory input manifestsitself in an alternating electrical field within the structure ofcochlea 15 (which electrical field appears and has been detected atround window 27). This electrical field (generated by hair cells 29) issensed by the nerve endings of the auditory nerve in basilar membrane 21and transmitted via the auditory nerve 16 to the cerebral cortex 17 ofthe brain which interprets such electrical signals as sound.

Where total loss of hearing results from sensory deafness, i.e., areduction in the sensitivity of the cochlea, no method or apparatuspresently exists for restoring the sensation of hearing. However, inaccordance with the present invention, it has been discovered that thesensation of hearing in people suffering from sensory deafness can beinduced by direct electrical excitation of the auditory nerve endingsdistributed along basilar membrane 21 within cochlea 15 by using abipolar electrode in the lower scala without the necessity of a complexencoding system such as disclosed in Doyle.

Referring now to FIGS. 4-6, direct electrical excitation of the auditorynerve endings within cochlea 15 is achieved by use of an intra-cochlearelectrode, generally designated 30. The body 31 of electrode 30 ismolded of a medically acceptable resilient material, such as silicone orother rubber or plastic material, which is of such a shape as to fitthrough round window 27 and into lower scala 25 of cochlea 15. Body 31of electrode 30 includes a notch 32 which is designed to fit the roundwindow margin and retain body 31 within lower scala 25 of cochlea l5.Electrode 30 further comprises a pair of gold or other suitable inertconductors 33 and 34 which are imbedded in and retained'by base member31. External leads 35 and 36 are connected to the ends of conductors 33and 34, respectively, whereby leads 35 and 36 supply electrical signalsto contacts 33 and 34, respectively. Body 31 may also include astiffening member (nto shown) imbedded therein, such as a strand ofwire, to obtain the desired degree of resiliency.

As described more fully hereinafter, and as shown in FIG. 6, electrode30 is inserted through round window 27 of cochlea 15 into lower scala 25where it extends along the basilar membrane for approximatelythreefourths of a turn thereof. According to the embodiment of FIGS.4-6, conductors 33 and 34 are positioned side-by-side, adjacent basilarmembrane 21, each of conductors 33 and 34 extending parallel to thelength of membrane 21. In addition, the shape of base member 31 is suchso as to provide a space between the surface 37 thereof oppositeconductors 33 and 34 and the wall of lower scala 25 to permitcirculation of the cochlea fluid through lower scala 25 as well as afluid escape path during insertion.

With an electrical excitation signal applied to conductors 33 and 34 vialeads 35 and 36, respectively, a uniform, alternating electrical fieldis generated therebetween. Because of the position of the conductors 33and 34 in the lower scala, the electrical field generated therebetweenis applied so as to allow place frequency selection to take place,unlike the Doyle system. This field is transmitted through theconductivecochlea fluid to the nerve endings in basilar membrane 21,thus replacing the naturally generated auditory electrical field. Theelectric field generated by conductors 33 and 34 is sensed by the nerveendings distributed along the basilar membrane 21 and conducted via theauditory nerve 16 to the cerebral cortex 17 of the brain whichinterprets such electrical signals as sound.

According to the preferred embodiment of the present invention, and asshown in FIGS. 4-6, conductors 33 and 34 are made from gold wire woundon a manderel. Conductors 33 and 34 so formed are inserted into notchesin base member 31 so as to slightly extend beyond the outer periphery ofbase member 31. The conductors are then imbedded within base member 31by filling such notches with additional resilient material. Three tofive strands of gold wire may serve as leads 35 and 36 for conducting anelectrical excitation signal to conductors 33 and 34. In addition, asexplained previously, conductors 33 and 34 extend parallel to the lengthof basilar membrane 21 for approximately threefourths of a turn. Thisafford conductive means along a substantial length of basilar membrane21, thereby exciting a relatively large frequency spectrum. Morespecifically, it has been found through experimentation that the nerveendings within basilar membrane 21 are frequency selective. The nerveendings adjacent round window 27 respond to frequencies at the high endof the audio spectrum and decrease in frequency sensitivity as the apex28 of cochlea 15 is approached. Accordingly, by extending conductors 33and 34 for a substantial length along basilar membrane 21, a relativelylarge frequency spectrum may be excited.

Once intro-cochlear electrode 30 is positioned within lower scala 25 ofcochlea 15, as will be explained more fully hereinafter, there must thenbe provided a means for coupling electrical signals to the conductorsthereof. The problem with inducing electrical signals within the cochleais, of course, the fact that no orifices are available for ready accessto the tympanic cavity.

In addition, the cochlea is well shielded within the heavy bonystructure of the skull. A direct electrical connection may be made withnormal wire conductive means but this introduces the risk of infection.The problem then becomes one of coupling electrical signals to theelectrode within the cochlea without the use of normal wire conductivemeans.

Referring now to FIG. 7, there is shown a preferred embodiment ofapparatus for exciting an intra-cochlear electrode. In the embodiment ofFIG. 7, the vibrations of the surrounding air are sensed by a microphone70 which converts the mechanical vibrations to an electrical signal inthe audio spectrum which is applied to a preamplifier 71. The output ofpreamplifier 71 is applied via a tone control network 72, to bedescribed more fully hereinafter, to a modulator 73. Modulator 73 isoperative to modulate the output of a combination oscillator/UHF.transmitter 74. The output of oscillator/transmitter 74 is applied to anantenna 75 which, in its preferred form, is an inductive coil. Thetransmitting network, consisting of elements 7075, may be mountedexternally of the body to sense the sound waves and convert such soundwaves into a modulated RF. signal. This modulated RF. signal is sensedby a receiving antenna 76, which may also be an inductive coil, andapplied to a reciever 77. The output of receiver 77 is demodulated by ademodulator 78 to restore the original audio excitation signal appearingat the output of tone control network 72. Finally, the output ofdemodulator 78 is applied to an intra-cochlear electrode 79.

The transmitting network consisting of elements 70 through 75 may haveany suitable configuration since elements 70-75 are positionedexternally of the body, as will be explained more fully hereinafter, andsize and complexity are not problems. On the other hand, sincecomponents 76-78 will be positioned internally of the body withelectrode 79, they should be as simple as possible. A preferredconfiguration for elements 7678 is shown in FIG. 8.

Referring now to FIG. 8, receiver 77 may comprise a tuned circuitconsisting of inductive coil 76 and a capacitor 81 tuned to thefrequency of oscillator/transmitter 74. In the case where modulator 73is an amplitude modulator, demodulator 78 may simply comprise a diodeconnected to one side of capacitor 81. The output of diode 82 may beshunted by a capacitor'83 and conducted via resistors 84 and 85 and alead 87 to one conductor of electrode 79. An additional diode 86 shuntsthe junction between resistors 84 and 85, the other side of capacitor 81being connected via a lead 88 to the other conductor of electrode 79. Insuch circuit, capacitor 83 and resistor 84 act as a filter and currentlimiter, respectively. Diode 86 is a noise limiting diode such thatnoise peaks occurring due to electromagnetic discharges can be limitedby forward conduction of diode 86. Resistor 85 also acts as a currentlimiting resistor in feeding the audio signal to electrode 79.

Referring now to FIG. 9, the physical configuration of a preferredembodiment of the present invention is shown. Intra-cochlear electrode79 is made integral with a continuous length 90 of medically acceptableresilient material in which leads 87 and 88 are imbedded. One end ofleads 87 and 88 are connected to the conductors within electrode 79whereas the other ends of leads 87 and 88 are connected to a smallintegrated cicuit chip or substrate 91 carrying the inductors,capacitors, resistors and diodes. Included with chip 91 is receivingcoil 76. Chip 91 is also imbedded within the resilient material. Thisentire sutructure, generally designated 92, would then be implanted invivo. A typical surgical procedure is as follows: The patient is placedon an operating table with the appropriate ear in a horizontal positionexposed in a sterile field. The auricle is folded forwardly and clampedin position and an incision made posterior to the ear. Entry to themiddle ear is gained by elevating the skin along the auditory meatuswhich permits a direct by-pass of the tympanic membrane. With the skinalong the auditory meatus and the tympanic membrane elevated, visualcontact may be made with the middle ear and the oval and round windowsat the entrance to the cochlea. A bony promontory protruding above theround window is then removed to permit free access to the round window.The round window membrane is then removed and a portion of the uppermargin excavated for easy access to the lower scala. Electrode 79 isthen inserted through the round window such that the electrodeconductors lay in close proximity to the basilar membrane between thelower scala and the scala media. Electrode 79 is then inserted into thelower scala until the notch therein slips into the round window margin.A channel is then excavated in the bony structure along the auditorycanal for location of material 90 containing leads 87 and 88, material90 then being sutured into position. The elevated skin along theauditory canal is then carefully returned to its original position andthe canal packed to assure proper adhesion.

The skin posterior to the incision is elevated and a small portion ofthe muscular structure attached to the skull removed to receive chip 91which is then sutured in place. A pair of test leads 93 and 94, as shownin FIG. 9, are connected directly to leads 87 and 88 and extendoutwardly from material 90 adjacent chip 91. Test leads 93 and 94 arebrought out through the incision. The transmitting network is thenactivated and a signal transmitted to receiver 77. The voltage acrossthe intra-cochlear electrode is monitored on an oscilloscope via testleads 93 and 94 to insure operability. After the electrode is tested andfound operative, test leads 93 and 94 are clipped and the incisionsutured. The external ear is then returned to its normal position andthe surgical procedure is completed.

With the surgical procedure completed, receiver 77 and antenna .76 inchip 91 are positioned immediately posterior the ear close-to and underthe skin. A unit may then be mounted behind the ear, such unit includingmicrophone 70, oscillator/transmitter 74 and transmitting antenna 75.The output of microphone may be conducted through electrical leads to apocketcarried unit containing preamplifier 71, tone control network 72,modulator 73 and a suitable power supply (not shown). The output ofmodulator 73 is then coupled back to the ear-mounted unit tooscillator/transmitter 74 and transmitting antenna 75. In this manner,transmitting and receiving antennas 74 and 76 will be positioned inclose proximity to each other, only a thin layer of skin separating thetwo elements. As a result, the current passing through antenna 74 isinduced in antenna 76 and applied to receiver 77.

With the elements so positioned, operation is as described previously vvith respect to FIGS. 7 and 8. In summary, the vibrations of thesurrounding air are sensed by microphone 70 and converted to anamplified, shaped, modulated R.F. signal by components 7174. Themodulated signal is transmitted by antenna 75 to antenna 76 wherereceiver 77 and demodulator 78 reproduce the original audio excitationsignal and apply it via leads 87 and 88 to electrode 79. With suchelectrical excitation signal applied to electrode 79, an electric fieldis generated between the conductors thereof. The electrical field sogenerated varies in amplitude proportioned to the pressure vibrations ofthe surrounding air, i.e., the audio signal to be heard. In other words,such field is an analog of the audio signal to be heard. The fieldgenerated between the conductors of electrode 79 is transmitted throughthe conductive cochlea fluid to the nerve endings in the basilarmembrane, thus replacing the naturally generated auditory electricfield. The electric field so generated is sensed by the nerve endingsdistributed along the basilar membrane and conducted via the auditorynerve to the cerebral cortex of the brain which interprets suchelectrical signals as sound.

Tone control network 72 is provided to shape the frequency spectrum ofthe signal applied to electrode 79, if desired. More specifically,initial tests with the present system have shown that it is not easyand, as a matter of fact, quite difficult, for a patient who has neverheard to properly interpret the electrical stimulus now being applied tothe auditory nerve endings. For this reason, it has been necessary toinitially shape the frequency spectrum applied to a particular patientto correspond to stimuli his brain is capable of interpreting. As thepatient gains experience in interpreting the signals applied to hiscochlea, the frequency spectrum of the applied signal is slowlyincreased. Accordingly, tone control 72 is inserted between preamplifier71 and modulator 73 to provide the desired shaping of the appliedexcitation signal.

The above-described system, including the intracochlear electrode shownin H6. 4 and the electronics described with respect to FIGS. 79 has beentested by implantation in selected patients at Sequoia Hospital, RedwoodCity, Calif. The surgery has been performed by Dr. Robin P. Michelson.In one such implant procedure, the surgical approach was identical tothat described hereinbefore. The patient was tested the following day bytransmitting signals to the receiver and then in turn to the electrode.The patient exhibited the ability to distinguish tones over thefrequency range 125 Hz to 4,000 Hz. His frequency discrimination at oneoctave steps from 250 Hz to 4,000 Hz was excellent. He exhibitedamplitude discrimination of pure tones with a calculated change of lessthan 2 db. The patient further exhibited a dynamic range, i.e.,thresholdto maximum listening level, of approximately 10 db. He wasgiven a Spondee Test and was able to recognize six of 35 words. Thepatients previous score on this test with a hearing aid was zero. Infurther tests on this patient, transmitting to the patient a randomseries of the numbers one through ten, the patient is presently capableof correctly distinguishing the numbers approximately 65 percent of thetime. In a second patient, where a similar implant procedure has beenperformed, such patient is capable of distinguishing a random series ofthe numbers one through 10 approximately 90-95 percent of the time.

In the tests which I have conducted on human patients l have found thatthe threshold of auditory perception under electrical stimulation is afunction of frequency. The voltage measured across the conductors of theintra-cochlear electrode which is required to stimulate auditoryperception increases as frequency increases. At t KHZ, three patientsexhibited a threshold of auditory perception of approximately 0.5 voltwhile at Hz, this threshold was observed to be approximately 0.1 volt.At higher frequencies, in the range of 2 to 5 KHZ, stimulation levels ashigh as 1 volt were required. l have found that the minimum electricalstimulation required for auditory perception in human patients is about005 volt. Thus, in order to induce hearing in a human subject, it isnecessary that at least 0.05 volt be impressed across the conductors ofthe intracochlear electrode of the present invention.

1 have recently conducted comparative tests between brain reception andelectrical or acoustical stimulation in cats and human subjects todetermine the stimulation level required to achieve equivalent hearingresults. The brain reception of electrical and acoustical stimulation incats was determined by recording the electrical response of the inferiorcolliculus, one of the higher hearing centers in the brain, sinceobviously a cat cannot relate to the investigator its level of auditoryperception. The tests demonstrated that cats appear to show identicalresponses from electrical and acoustical stimulation when stimulated atthe same level as human patients. Since the minimum auditory perceptionthreshold in humans is approximately 0.05 volt, it can be deduced that acomparable minimum perception level occurs in cats. Because the maximumelectrical stimulation utilized in my previous tests described in theaforementioned Michelson article entitled The Crossed Cochlea Effect was250 microvolts, it is apparent from the comparative tests discussedhereinbefore that the cats used in the previous tests could not hear atthe levels of stimulation utilized.

Therefore, and in accordance with the present invention, there isdisclosed a method for inducing the sensation of hearing by directelectrical stimulation of the auditory nerve endings of the auditorynerve. Since the present technique completely by-passes the external andmiddle ears and most of the internal ear other than the basilarmembrane, it may be effectively used to induce the sensation of hearingin people suffering from deafness caused by abnormalities in any ofthese areas. However, the primary use will be in the case of sensorydeafness which has, heretofore, been untreatable.

In accordance with the present invention, the sensation 'of hearing isinduced by positioning an intracochlear electrode within the lower scalaof the cochlea, such electrode being surgically inserted through theround window. The electrode includes a pair of conductors which extendparallel to the length of the basilar membrane. Means are disclosed fortransmitting an excitation signal to a receiver implanted with andconnected to the conductors. Such excitation signal creates a uniform,alternating electrical field between the conductors, which electricalfield is transmitted through the conductive cochlea fluid to the nerveendings in the basilar membrane, thus replacing the naturally generatedauditory electric field. The electric field so generated is sensed bythe nerve endings distributed through basilar membrane 21 and conductedvia the auditory nerve to the cerebral cortex of the brain whichinterprets such electrical signals as sound.

While the invention has been described with respect to the preferredphysical embodiment constructed in accordance therewith, it will beapparent to those skilled in the art that various modifications andimprovements may be made without departing from the scope and spirit ofthe invention. For example, although the preferred embodiment of thepresent method for generating an alternating electric field along thebasilar membrane within the cochlea utilizes an electrode adapted to bepositioned within the lower scala of the cochlea, it will be appreciatedby those skilled in the art that it is theoretically possible, althoughnot presently practical, to position an electrode within the upper scalaor the middle scala of the cochlea. Accordingly, it is to be understoodthat the invention is not to be limited by the specific illustrativeembodiments, but only by the scope of the appended claims.

I claim:

1. A method for inducing the sensation of hearing on human subjectscomprising:

generating an alternating electrical field along the basilar membranewithin the cochlea, said electrical field being an analog of an audiosignal to be heard.

2. The method of claim 1 wherein the step of generating an alternatingelectrical field along the basilar membrane comprises:

positioning an electrode within the cochlea, said electrode including abase member and a pair of conductors retained by said base member; andconducting an electrical excitation signal which is an analog of saidaudio signal to said conductors.

3. The method of claim 2 wherein a potential of not less than about .05volts is impressed across said conductors.

4. A method for inducing the sensation of hearing in human subjectscomprising:

generating a uniform, alternating electrical field along a substantialportion of the basilar membrane within the cochlea, said electricalfield being an analog of to an audio signal to be heard.

5. The method of claim 4 wherein the step of generating a uniform,alternating electrical field along the basilar membrane comprises:

positioning an electrode within the lower scala of the cochlea, saidelectrode including a base member and a pair of elongated conductorsretained by said base member, said conductors adapted to extend parallelto the length of the basilar membrane; and conducting an electricalexcitation signal corresponding to said audio signal to said conductors.

6. The method of claim 5 wherein a potential of not less than about 0.05volts is impressed across said conductors.

7. A method for inducing the sensation of hearing in human subjectscomprising:

positioning an electrode within the lower scala of the cochlea, saidelectrode including a resilient base member and a pair of conductorsretained by said base member; and

conducting an electrical excitation signal which is an analog of anexternally generated audio signal to said conductors.

8. The method of claim 7 wherein each of said conductors comprises anelongated member adapted to extend parallel to the length of the basilarmembrane, said conductors being positioned side-by-side in said basemember, immediately adjacent said basilar membrane.

9. The method of claim 7 wherein a potential of not less than 0.05 voltsis impressed across said conductors.

1. A method for inducing the sensation of hearing on human subjectscomprising: generating an alternating electrical field along the basilarmembrane within the cochlea, said electrical field being an analog of anaudio signal to be heard.
 2. The method of claim 1 wherein the step ofgenerating an alternating electrical field along the basilar membranecomprises: positioning an electrode within the cochlea, said electrodeincluding a base member and a pair of conductors retained by said basemember; and conducting an electrical excitation signal which is ananalog of said audio signal to said conductors.
 3. The method of claim 2wherein a potential of not less than about .05 volts is impressed acrosssaid conductors.
 4. A method for inducing the sensation of hearing inhuman subjects comprising: generating a uniform, alternating electricalfield along a substantial portion of the basilar membrane within thecochlea, said electrical field being an analog of to an audio signal tobe heard.
 5. The method of claim 4 wherein the step of generating auniform, alternating electrical field along the basilar membranecomprises: positioning an electrode within the lower scala of thecochlea, said electrode including a base member and a pair of elongatedconductors retained by said base member, said conductors adapted toextend parallel to the length of the basilar membrane; and conducting anelectrical excitation signal corresponding to said audio signal to saidconductors.
 6. The method of claim 5 wherein a potential of not lessthan about 0.05 volts is impressed across said conductors.
 7. A methodfor inducing the sensation of hearing in human subjects comprising:positioning an electrode within the lower scala of the cochlea, saidelectrode including a resilient base member and a pair of conductorsretained by said base member; and conducting an electrical excitationsignal which is an analog of an externally generated audio signal tosaid conductors.
 8. The method of claim 7 wherein each of saidconductors comprises an elongated member adapted to extend parallel tothe length of the basilar membrane, said conductors being positionedside-by-side in said base member, immediately adjaceNt said basilarmembrane.
 9. The method of claim 7 wherein a potential of not less than0.05 volts is impressed across said conductors.